In many apparatuses, such as pumps, a rotational coupling includes a first component coupled to a second rotating component. For instance, a pump may include a rotational coupling that includes a rotor, or driven member, coupled to a rotating shaft via a coupling. Often, the coupling includes a pin that extends through a pin bore defined by the shaft. The rotating shaft and the pin extending through the shaft are received in an opening defined by the rotor. Thus, when the shaft is rotated, a portion of the pin on each side of the shaft makes contact with the rotor and drives the rotor to rotate with the shaft. Depending on the pump, the rotation of the shaft can be powered by various means, including but not limited to, an electric motor.
Often, in order to reduce manufacturing costs, the rotor is comprised of a relatively weak material, such as powered metal. Those skilled in the art will appreciate that components comprised of powered metal have been proven to be relatively simple and economical to manufacture. For instance, the rotor can be manufactured by consolidating the metal in its powered state into a mold or die having the desired shape of the rotor. The mold or die can then be heated in order to strengthen the powered metal. To further reduce manufacturing costs without decreasing the strength of the rotational coupling, the pin extending through the shaft is often machined to be cylindrical. The cylindrical pin and a cylindrical pin bore are relatively simple and economical to manufacture. Further, because that the cylindrical pin bore can be machined to be relatively small, the cylindrical pin bore does not unacceptably decrease the strength of the shaft.
Although the rotational coupling including the powdered metal rotor and the cylindrical pin has been found to be relatively economical to manufacture, there is room for improvement. When the shaft is rotated, drive surfaces of the pin located on opposite sides of the shaft make contact with contact surfaces of the rotor in order to drive the rotor. Because the pin is cylindrical, the drive surfaces contacting the contact surfaces are relatively thin. Thus, the driving force required to rotate the rotor is distributed over a relatively small load contact area between the pin and the rotor. Those skilled in the art will appreciate that stress caused by the driving force is equal to the driving force over the area on which the driving force acts. Thus, the smaller the contact area, the greater the stress caused by the driving force. Moreover, because the contact surfaces of the rotor are comprised of powdered metal, a relatively weak material, the contact surfaces may have a reduced tolerance to wear caused by the stress of the driving force acting on the contact surfaces. The wear of the rotor may lead to premature failure of the rotational coupling.
The wear on the rotational coupling has been found to be particularly unacceptable in situations in which the rotational coupling is rotating against relatively high pressures. For instance, in some fuel transfer pumps, the rotational coupling can be used to transfer fuel through the pump. The rotational coupling can be positioned within a cavity defined by a cam ring such that the rotational coupling and the cavity are eccentric, meaning that each has a different center. Thus, a clearance defined by an outer surface of the rotor and an inner surface of the cam ring will vary in size around the circumference of the rotational coupling. When the rotor rotates, rollers driven by the rotor can make contact with the inner surface of the cam ring and push the fuel within the clearance around the circumference until the fuel reaches a minimum clearance area. The high pressure created at the minimum clearance area will push the trapped fuel out of the clearance and through an outlet port of the pump. As the fuel is pushed around the circumference by the rollers, a void is created within the clearance. Low pressure created by the void will draw more fuel into the clearance for circulation, and the process will repeat itself.
In order to increase the fuel pressure being delivered from the pump, the rotational speed of the shaft, and thus the rotor, is increased. The faster the rotor is rotated, the greater the pressure within the clearance against which the driving force must rotate the rotational coupling. The greater the driving force, the greater the stress acting on the rotor causing wear and possibly premature failure. Thus, the problems associated with wear, especially rotor wear, caused by stress is increased at higher pressure applications.
Although engineers have found that the wear on the rotor can be decreased by comprising the rotor from a relatively strong material, such as steel, the costs and difficulty associated with manufacturing a steel rotor may be unacceptably high. Moreover, although the wear on the rotor may be decreased by altering the cylindrical shape of the pin to include a larger drive surface, the manufacturing costs associated with the machining of the pin and pin bore in a shape other the cylindrical may also be unacceptably high. In addition, a larger pin bore may decrease the strength of the shaft.
It should be appreciated that the above discussed wear problems may be present in rotational couplings used within apparatuses other than pumps. The present invention is directed at overcoming one or more of the problems as set forth above.